From: Chuck Reeves (creeves_at_windows.microsoft.com)
Date: Wed Jan 21 2004 - 10:34:59 PST
The paper, "Application Performance and Flexibility on Exokernel
Systems" was submitted to SOSP in 1997 and was written by a number of
researchers and MIT. It describes their experiences in building and
testing a number of exokernel operating systems.The exokernel design
proposes to expose a higher level of control and more information
relative to hardware and software resources to untrusted software. In
this model, management of these resources is available to user
applications, but protection is still guaranteed by privileged software
running in the kernel. In the designs presented here, the additional
facilities are leveraged by constructing unprivileged library operating
systems. Through the use of the additional information exposed by the
kernel, these libraries are able to provide better, more appropriate
abstractions for managing frequently-used, expensive resources. Most of
the examples, revolve around improvements recognized in managing disk
I/O.
Section 4 of the document describes the design of a disk management
system called XN. It's use of UDF's (untrusted deterministic functions)
as a common abstraction for manipulating ownership and access control
seemed to enable flexibility in the layout information on disk. The
authors also discuss some of the details of a file system called C-FFS
that they built on top of XN.
Section 5 details Xok and ExOS, an exokernel and it's associated default
library. I found the idea of providing per-page transformations to
compress or digitally sign information at an application level an
interesting indicator of the level of control this approach enables.
Much of the rest of the document is committed to comparisons of
measurements taken on the performance of applications running on top of
Xok/ExOS to those on commercial UNIX offerings. The results do seem to
confirm that in disk intensive applications, a finer level control can
result in improved performance.
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